Identification of regions in a formation that contain hydrocarbons is the primary goal of oil and gas exploration. The hydrocarbon containing regions are often referred to as pay regions of the formation. One way to identify the pay regions is based on the resistivity of the formation at different depths.
Formation resistivity may be measured with electrodes (laterologs) or antennas (induction logs) that can transmit a current or electromagnetic (EM) energy into earth formations. The energy transmitted into the formations interacts with the conductive media in the formations. With laterologs, a current (or voltage) is injected into the formation using a first pair of electrodes. A second pair of electrodes is typically placed at a distance from the first pair of electrodes to measure the voltage drop or current flow between the second pair of electrodes. The measured voltage drop or current flow may be used to derive the resistivity (or its inverse, conductivity) of the formation. In this description, resistivity is intended to include its inverse, conductivity, and vice versa, because embodiments of the invention are equally applicable to resistivity or conductivity.
With induction logging, EM energy is transmitted into a formation to induce eddy currents in the formation. The eddy currents flow in loops that lie on planes perpendicular to the magnetic dipole of the transmitting antenna. The magnitudes of the eddy currents depend on the conductivities of the formation. The eddy currents in turn induce secondary magnetic fields, the magnitudes of which depend on the magnitudes of the eddy currents. Therefore, by measuring the magnitudes of the secondary magnetic fields (using a receiver antenna), it is possible to indirectly determine the resistivity of the formation around the transmitter and receiver antennas.
Resistivities of formations depend on the amounts and types of fluids included in the pores therein. Thus, different formations may have different resistivities due to different porosities, and/or different amounts or types of fluids included therein. When the formation is homogeneous, the electric property (resistivity or its inverse, conductivity) is constant regardless of the direction of the measurements. However, earth formations often include sedimentation layers that may have different geophysical properties (e.g., grain sizes, porosities, etc.), and hence different electrical properties. For example, the resistivity of shale may be different in different directions. Thus, the formation may have a resistivity property that differs in different directions. This phenomenon is referred to as formation (electrical) anisotropy.
In a typical situation, a borehole may be drilled through multiple sedimentation layers in a direction perpendicular to the layers, i.e., a vertical well. In a vertical well, a resistivity measurement along a direction parallel the borehole axis is referred to as a vertical resistivity because the measurement is made in a direction perpendicular to the sedimentation layers. In the vertical resistivity measurements, the current paths run through various sedimentation layers, which act like different resistors connected in a series.
FIG. 1 shows an example model of a formation (10) with a borehole (12). As shown in FIG. 1, the formation (10) may have multiple layers of shale (14) and sand (18). The shale (14) in the formation has an anisotropic resistivity property. Specifically, as shown by the shale resistivity values (16), the vertical resistivity of the shale (Rshale-v) in the example formation (10) is 2 ohms meter (Ω·m) while the horizontal resistivity of the shale (Rshale-h) is 1 Ω·m. In contrast, the sand (18) in the formation (10) has an isotropic resistivity property. In the example, the resistivity of the sand (Rsand) in both the horizontal direction and the vertical direction is 20 Ω·m as shown by the sand resistivity values (20). The shale fraction is 0.4 while the sand fraction is 0.6 as shown by the shale and sand fraction values (22).
Continuing with FIG. 1, the formation (10) exhibits anisotropic resistivity property because of the shale anisotropy and the sand and shale fraction values (24). In particular, the vertical resistivity (RV) in the example formation (10) is 12.8 Ω·m while the horizontal resistivity (RH) is 2.3 Ω·m. By identifying the horizontal and the vertical resistivity, the pay (i.e., hydrocarbon yielding) region and the non-pay regions of the formation (10) may be identified.
Over the years, most of the homogeneous or thick-layer oil and gas reservoirs have been discovered. As a result, many reservoirs comprise thin layers of pay regions. With technology advances such as directional and horizontal drilling, it is becoming economical to produce in thin reservoirs that traditionally would have been ignored. The industry has also begun to realize the importance of thinly laminated reservoirs that have been by-passed due to low apparent resistivity in vertical wells. Therefore, a need exists for methods that can accurately identify the pay regions of the formation.